The FDA has no power to levy taxes. The US Senate and House of Representatives have that power set aside. "Deeming" applies only to whether or not a specified product is covered under the law. Right now the only covered products are cigarettes, roll-your own, and smokeless. But the law says that if the FDA would like to regulate additional products, all the FDA has to do is to issue a "deeming regulation" which proclaims that henceforth the FDA has the authority to regulate the specified product(s) under the Tobacco Act.
A deeming regulation does not get into specific rules for the products. All it does is proclaim that the products are going to be regulated by the FDA, and that in the future any rules the FDA finalizes must be followed by the regulated companies.
If the FDA does what we fear it will do, there won't be any product on which to levy taxes, even if the FDA could tax things.
Basically we fear that they will apply certain provisions of the law that were designed to deal with combustible cigarettes to these safer alternatives. For example, the law specified that the FDA could ban tobacco cigarettes that have a "characterizing flavor" (e.g., "vanilla.") Another provision that states that any product that was not actively being sold before Feb. 15, 2007 must be taken off the market until approved as a "new tobacco product" or has an application approved to be considered substantially equivalent to another product that WAS being sold before Feb 15, 2007. Note that the deadline for submitting "substantial equivalence" applications has already come and gone, and the FDA has not acted on even one of these applications that were submitted by Lorillard, RJ Reynolds, and Altria.
The results would be either a complete ban on all electronic cigarettes for years and years, or allowing a few of the earliest models (most of which are not even manufactured any more) to stay on the market. These were much less effective than current models.
Assuming some models are left on the market, they can write regulations that would make them much less effective, such as outlawing flavors, setting too-low concentration levels for nicotine, and tinkering with the hardware side of things.